Simulating the Gravitational Displacement of a Gigantic Manipulator Posture Using Lumped Stiffness-Matrix Modeling on a Physics Engine

The ITER project seeks to construct the world's largest fusion device to validate fusion as a viable energy source. The blanket remote handling system (BRHS) manages blanket modules weighing up to four tons, which shield the vessel from fusion plasma. A virtual reality (VR) environment is indispensable for effective motion planning and monitoring during BRHS operations. However, gravitational displacements over 100 mm at the end -effector pose a challenge to achieving the required ±50 mm error range of the VR relative to the actual manipulator. To address this issue, we propose a method utilizing the lumped model, in which virtual joints simulating the stiffness of the manipulator links are implemented. The physics engine of the VR software calculates the deformation of the virtual joints. Since stiffness parameters are obtained through structural analysis, this approach eliminates the need for actual equipment measurements. The virtual joints feature a six-degree-of-freedom (6-DoF) spring, commonly used in the lumped model, and a full stiffness matrix for better simulation. Through evaluation, we reduced the maximum VR error from 97.8 mm (rigid model) to 64.6 mm (6-DoF spring model). Although the maximum error still exceeded the ±50 mm requirement, the proposed method significantly improved the VR accuracy.